1. Field of the Invention
The present invention relates to a DC offset canceling circuit applied in a variable gain amplifier, and particularly to a DC offset canceling circuit which uses internal capacitors inside an IC for generating a filtering function.
2. Description of Related Art
Variable gain amplifiers (VGA), which amplify input signals to necessary voltage levels of a system in demodulation process, are largely used in home network transceivers which transmit signals via cables. When the variable gain amplifier is used, a differential input end of an internal operational amplifier has the problem of intrinsic offset, and the intrinsic offset is always in the range of several mV to tens of mV. For wireless or wire communication, the maximum gain of the variable gain amplification is up to tens of dB; therefore, the intrinsic offset after amplification will affect the recovery ability of the received signal the characteristics of parameters of a dynamic range and signal noise ratio. A DC offset canceling circuit is shown in FIG. 1 (a), disclosed by A. Parssinen et al., in xe2x80x9cA 2-GHz Wide-Band Direct Conversion Receiver for WCDMA Application,xe2x80x9d IEEE J.Solid-State Circuits, Vol. 34, pp. 1893-1903, December 1999. In FIG. 1, a closed loop feedback path 11 is formed by an operational amplifier, resistors and capacitors to cancel the intrinsic offset. Since the closed loop feedback path 11 executes the function of a low pass active-RC filter, the capacitors Cext 13 are huge and have to be put outside the chip. For designing a chip, it is necessary to provide more I/O pins to electrically connect to the external capacitors Cext 13. Consequently, both the design complexity and cost are increased.
Another prior art DC offset canceling circuit is shown in FIG. 1(b), disclosed by C. Dennis Hull et al., in xe2x80x9cA Direct Conversion Receiver for 900 MHz (ISM Band) Spread-Spectrum Digital Cordless Telephone,xe2x80x9d IEEE J. Solid-State Circuits, Vol. 31, No. 12, pp. 1955-1963, December 1996. The circuit is formed by cascading an operational amplifier, resistors and capacitors, and a closed loop feedback path 12 is formed to cancel the intrinsic offset. As mentioned above, the external capacitors Cext 13 contained in the closed loop feedback path 12 are huge and must be put outside the chip. For designing a chip, it is necessary to provide more I/O pins to electrically connect to the external capacitors. Consequently, both the design complexity and cost are increased.
A first object of the present invention is to cancel the intrinsic offset of a variable gain amplifier.
A second object of the present invention is to provide a DC offset canceling circuit having less I/O pins.
A third object of the present invention is to provide a simplified DC offset canceling circuit at a lower cost.
For achieving the above objects, the present invention proposes a DC offset canceling circuit applied in a variable gain amplifier, which comprises a transconductance amplifier and at least one internal capacitor to function as a filter. The input of the transconductance amplifier is electrically connected to the output of the variable gain amplifier, and the output of the transconductance amplifier and the at least one internal capacitor are electrically connected to the input of the variable gain amplifier to form a feedback loop. To cooperate with the function of the DC offset cancelation, the input stage of the variable gain amplifier comprises an auxiliary differential pair.
The DC offset canceling circuit applied in a variable gain amplifier according to the present invention comprises a transconductance amplifier, at least one internal capacitor and an auxiliary differential pair. The transconductance amplifier is electrically connected to the output of the variable gain amplifier for transforming an input voltage to an output current based on a ratio. The at least one internal capacitor is electrically connected to the output of the transconductance amplifier for generating a filtering function by working together with the transconductance amplifier. The auxiliary differential pair is situated at the input of the variable gain amplifier and electrically connected to the output of the transconductance amplifier to function as a current switch. Besides, the variable gain amplifier comprises a first amplifier to function as an input stage, and the auxiliary differential pair is embedded in the input end of the first amplifier. The transconductance amplifier and the at least one internal capacitor are embedded in a chip.